The present invention relates to a circuit arrangement for connecting a subscriber to a bus line.
In systems in which information is transmitted between several subscriber stations, bus lines are frequently used, to which the subscriber stations are connected in parallel. The stations may be information sources, sinks or combined sources/sinks. For connecting the stations to the bus line, transmitters, receivers or combined transmitter/receiver modules are employed. While the receivers can be connected to the bus line at a high impedance level via resistors, i.e., passive components, and therefore, in the event of a short circuit between a signal conductor and another signal conductor, to ground or against a supply voltage, the signal level on this short-circuited conductor is not influenced detrimentally, the transmitters are connected to the bus line at a low impedance level, which, in case of a short circuit, leads to the situation that at least one conductor of the bus line is connected to constant potential and the bus line is thereby blocked. The transmission of data between the subscriber stations is then no longer possible, whereby practically the entire system has failed. To avoid this problem, it is known from DE-AS No. 26 47 367 to design the bus line and the connections of the subscriber stations redundantly. With such an arrangement, the main advantage of bus systems, namely, the relatively low cost, thereby is reduced, however.
In U.S. Pat. No. 4,236,087, the problem is described that in bus systems, one defective subscriber station can block the whole system. In order to avoid this, the defective station is electrically separated from the data bus. To this end, the line drivers are switched in the event of a distrubance, into the blocked state by an isolation control register. With such a circuit, the station can then be separated effectively from the bus line if the defect is, for instance, in the control of the circuit. In the event of a defect of a line driver, for instance, a short to ground, the circuit is inoperative, however. Also, the line drivers are blocked only if a defect has been recognized.